1. Field
Example embodiments generally relate to fuel structures used in nuclear power plants.
2. Description of Related Art
Generally, nuclear power plants include a reactor core having fuel arranged therein to produce power by nuclear fission. A common design in U.S. nuclear power plants is to arrange fuel in a plurality of cladded fuel rods bound together as a fuel assembly, or fuel bundle, placed within the reactor core. These fuel bundles typically include several spacing elements placed axially throughout the bundle to dampen vibration of the fuel rods, ensure minimum separation and relative positioning of the fuel rods, and mix coolant flowing axially through the bundle and spacers therein.
As shown in FIG. 1, a conventional fuel bundle 110 of a nuclear reactor, such as a BWR, may include an outer channel 112 surrounding an upper tie plate 114 and a lower tie plate 116. A plurality of full length fuel rods 118 and/or part length fuel rods 119 may be arranged in a matrix within the fuel bundle 110 and pass through a plurality of spacers 120 axially spaced one from the other and maintaining the rods 118, 119 in the given matrix thereof. The fuel rods 118 and 119 are generally continuous from their base to terminal, which, in the case of the full length fuel rod 118, is from the lower tie plate 116 to the upper tie plate 114.
FIG. 2 is an illustration of a grid-type spacer 121. As used herein, a specific “-type” spacer generally refers to all spacers having substantially similar hydraulic and operational characteristics, regardless of other minor variations in shape, size, rod matrix number, etc. As shown in FIG. 2, grid-type spacers 121 are a welded metal lattice divided into several boxes or cells 127 through which fuel rods 118/119 may pass. Grid-type spacers may frictionally grip to the fuel rods through the use of resistive contact segments 122, known as stops and/or springs, abutting the exterior of each rod that passes through the spacer 120. Mixing tabs and/or swirl vanes 123 may extend from the spacer 121, in order to better mix a coolant/moderator flowing through the spacer 121 and fuel rods extending there through. Example embodiment grid-type spacer 121 may be modified in several ways and still be considered a grid-type spacer. For example, gaps for water rods in the spacer may be resized, reshaped, or omitted. Or for example, the number and position of boxes 127 may be varied depending on the fuel assembly dimensions and characteristics. These variations may have a negligible effect on the overall hydraulic properties of example embodiment grid-type spacers, permitting them to remain classified as grid-type spacers.
FIG. 3 is an illustration of a ferrule-type spacer 125, including several ferrules 126 arranged in a grid. Each ferrule 126 may elastically fit around the circumference of a fuel rod 118/119, allowing less contact between a fuel rod 118/119 and spacer 125 and/or providing a less-restrictive flow path for a liquid film flowing through gaps between fuel rod 118/119 and spacer 125. Example embodiment spacers 121 and 125 may be held stationary at constant axial positions within the fuel bundle as high velocity coolant flows axially through the bundle 110 and may maintain fuel rods 118/119 in a static orientation within a fuel bundle. Example embodiment ferrule-type spacer 125 may be modified in several ways and still be considered a ferrule-type spacer. For example, gaps for water rods in the spacer may be resized, reshaped, or omitted. Or for example, the number and position of ferrules 26 may be varied depending on the fuel assembly dimensions and characteristics. These variations may have a negligible effect on the overall hydraulic properties of example embodiment ferrule-type spacers, permitting them to remain classified as ferrule-type spacers.
Existing method of measuring the force applied by spacers to the fuel rods requires disassembling the fuel bundle. This process cannot account for affects that rods may experience when the bundle is assembled and not experienced when disassembled. Disassembly may also bias spring force measurements. Additionally, disassembly processes are time consuming, and therefore costly. Also, this serves as an impediment to channeled fuel shipping.